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IV Characteristics
Electricity Lesson 4
Learning Objectives
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To recall the symbols for different circuit
components.
To investigate the characteristics of different
components.
Draw and explain the characteristics for a wire, a
filament bulb, thermistor and diode.
Explain how and why resistance depends on
temperature for metals and semiconductors.
Circuit Symbols?
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Ammeter
Voltmeter
Cell
Indicator or Light
Source
Diode
Light Emitting Diode
Resistor
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Variable Resistor
Thermistor
Light Dependent
Resistor
Heater
Electric Motor
Complete the Table
V
(a)
(b)
12 V
(c)
14.4 V
I
R
0.30 A
18 Ω
88 Ω
0.52 A
Ammeter
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An ammeter measures the current through part
of the circuit. It is always connected in series.
Voltmeter
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A voltmeter measures the potential difference
between two points. It is always connected in
parallel.
Cell
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A cell provides voltage to a circuit. This pushes
the charge around the circuit.
Lamp
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The symbol for an indicator or any light source
is the same (not including a light emitting diode).
Diode
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A diode only allows current to flow in one
direction. (In the way the triangle points) The
diode has a very high resistance in the reverse
direction. (For LED add the two little arrows)
Resistor
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A resistor impedes the flow of charge around a
circuit. As it does so, it heats up.
Variable Resistor
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A resistor that's value can easily be changed.
Thermistor
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The resistance of a thermistor decreases as the
temperature increases.
Light Dependent Resistor
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An LDR is a light dependent resistor. Its
resistance decreases as the amount of light
falling on it increases.
Investigating I-V Characteristics
Wire (Ohmic Conductor)
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For a resistor, the
current-voltage graph
is a straight line
through the origin.
The current through a
resistor is directly
proportional to the
potential difference
(voltage) across the
resistor.
Ohmic Conductor
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Note that for an Ohmic conductor the gradient
of the line an I-V graph is equal to 1/R.
V
I
R
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But this is for an Ohmic conductor only.
This is because R for a metallic conductor is
constant under constant physical conditions.
It does not apply for the filament bulb.
Filament Bulb
Filament Bulb
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For a filament lamp, as the voltage increases, so
does the current.
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But the increased current raises the temperature
of the filament bulb which also increases the
resistance.
This is why the curve bends toward the end as it
is increasingly difficult for more current to flow.
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Thermistor
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At constant T, it gives
a straight line.
The higher T is, the
greater the gradient of
the line as the
resistance falls with
increasing T.
Diode
Diode Notes
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The diode only allows current to flow in one
direction (forward direction).
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The diode has a high resistance in the opposite
direction (reverse direction).
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Needs a certain pd to conduct (typically about
0.6 V for silicon diode.)
Applications
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Why does the resistance of metals increase with
temperature?
Positive temperature coefficient.
Thermistor  Temperature sensors
Negative temperature coefficient.
% change of resistance.
Why?  number of charge carriers.